In a conventional method of forming a cam lobe, a substantially columnar blank is axially pressed by hot forging, so that a barrel-shaped primary intermediate forged article is formed. Then, a preliminary hole, which has a substantially circular sectional shape that is perpendicular to the shaft line, is punched out in a shaft center of the primary intermediate forged article, so that a secondary intermediate forged article is formed. Thereafter, a plug is pressed into the preliminary hole of the secondary intermediate forged article to fill the preliminary hole. The secondary intermediate forged article is again hot-forged, and a tertiary intermediate forged article that is close to a finished cam lobe is formed. Following thereto, the plug is drawn out from the tertiary intermediate forged article, so that a finished cam-lobe hole is formed. A desired cam-lobe contour is finished by cutting, whereby the finished cam lobe is formed (see, JP8-90139A (Patent Document 1)).
In another conventional method of forming a cam lobe, a substantially columnar blank is axially pressed by cold forging, so that an intermediate forged cam lobe having a burr on a contour thereof is formed. Then, a preliminary hole, which has a substantially circular sectional shape that is perpendicular to the shaft line, is punched out in a shaft center of the intermediate forged cam lobe. At the same time, the burr on the outer circumference of the intermediate forged cam lobe is punched out. In addition, the outer circumference of the intermediate forged cam lobe is further forged, so that an excess material extends into the preliminary hole. After that, a cam-lobe hole, which has a substantially circular sectional shape that is perpendicular to the shaft line of the intermediate forged cam lobe, is punched out in the shaft center of the intermediate forged cam lobe, whereby a finished cam lobe is formed (see, JP2006-169961A (Patent Document 2)).
In the method of forming a cam lobe described in Patent Document 1, the preliminary hold of the secondary intermediate forged article is filled by pressing thereinto a plug, and then the secondary intermediate forged article is further forged. Thus, the blank material cannot flow into the preliminary hole upon the further forging, but flows only along the outer circumference. This invites a high forging load.
On the other hand, in the method of forming a cam lobe described in Patent Document 2, in order to prevent generation of an underfill on the contour of the intermediate forged cam lobe, a large burr is left on the outer circumference of the intermediate forged cam lobe. That is, after the burr has been left on the outer circumference of the intermediate forged cam lobe in a forging-die, the blank material cannot move inside the forging-die, which also invites a high forging load.